1. Field of the Invention
The invention relates to spinal fusion surgery and, more particularly, to a clamp useful in such surgery.
2. Description of the Prior Art
Spinal fusion is performed to prevent motion between mobile segments of the spine. A variety of reasons exist for performing spinal fusion. The spine may be unstable due to a traumatic injury, surgery, or invasion and destruction of the vertebrae by tumor. Continued motion of particular segments of the spine may cause overgrowth of joint and ligamentous tissue which, in turn, may compress the spinal cord or its nerves. The curvature of the spine may become abnormal and cause deformity or neurological problems. In these instances, it may be desirable to prevent spinal motion at the affected levels.
The spine is composed of individual bones, or vertebrae, stacked on top of each other in a column. Each vertebra includes a cylindrical vertebral body, which participates in weight bearing, and an arch of bone (comprising the lamina and spinous process) which protects the spinal cord and its coverings. The bony arch is connected to the vertebral body by two small columns of bone, referred to as the pedicles. The circular canal between the body, the arch, and the pedicles houses the spinal cord and is called the spinal canal. Between adjacent vertebral bodies lie the intervertebral discs. These are cartilaginous structures that function as shock absorbers for the spine. Facet joints connect the bony arches of the spine and permit spinal motion between adjacent vertebrae.
Spinal instrumentation is employed as an adjunct to successful spinal fusion. The instrumentation immobilizes the spine while the body forms new, solid bone. Spinal fusion usually is performed by surgically exposing the area of the spine to be fused and thereafter preparing the exposed bone by removing soft tissue and ligaments so new bone can form over the area. After the surgical site has been prepared, an autogenic bone graft (from another part of the body, usually the hip) or an allogenic bone graft (from a cadaver) can be implanted in the prepared area so that new bone can form around and within the implant. Recently, non-biologic implants have been developed in an attempt to avoid the problems associated with acquiring a bone graft implant. Regardless of the type of implant that is used, the chances of achieving a successful fusion are enhanced if motion in the area is minimized or prevented while new bone forms. Since it usually takes from three to 12 months for new bone to form, motion in the affected area must be minimized or prevented for approximately three to 12 months.
Motion during the bone formation period may be prevented by wearing a hard external plastic brace or by installing internal metal instrumentation. Instrumentation usually entails placing screws, hooks, or wires in the affected vertebrae, pedicles, laminae, or spinal joints and then connecting the screws, hooks, or wires to metal rods or a metal plate. The rods or plate act as a solid bridge between the vertebrae, and the screws, hooks, or wires secure the bridge to the vertebrae. This technique reduces or eliminates motion between the vertebrae while bony fusion occurs. An example of a spinal fusion system of the type described is shown by U.S. Pat. No. 5,611,800.
Depending on the area of the spine being fused, the addition of instrumentation may substantially increase the chance of successful fusion as opposed to only wearing an external brace. Spinal instrumentation also may afford immediate spinal rigidity and allow the patient to return to normal physical activity sooner than otherwise possible. The benefits of instrumentation are offset by the potential for problems to occur due to the installation or failure of the instrumentation itself. Surgical procedures involving the placement of instrumentation often require significant time and skill to perform. Risk of injury to nerves or blood vessels in the fusion area may be higher than if instrumentation were not used. The instrumentation may break or cause pain and subsequently may need to be removed. Although the use of instrumentation usually outweighs the risks of not using it, it is desirable to employ the most easy-to-install, reliable form of instrumentation in order to increase the chances that the procedure will be effective.
Desirably, a technique for spinal fusion would be known that would enable two or more vertebral bodies to be joined quickly and easily. Preferably, such a technique would not require the use of screws, wires, or hooks.
In response to the foregoing concerns, the present invention provides a new and improved technique for spinal fusion. The invention employs a spinous process clamp in which a pair of elongate plates are positioned on either side of the spinous processes of vertebrae that are to be fused. The plates are joined by fasteners, preferably bolts and nuts. When the bolts and nuts are tightened, the spinous processes are clamped between the plates, thereby preventing motion between the clamped vertebral bodies.
In an alternative embodiment of the invention intended particularly to immobilize the lumbar region of the spine, projections are provided to extend from the lower end of the plates. The projections engage the upper portion of the patient""s sacrum and stabilize it relative to the vertebral bodies. If desired, brackets can be provided to connect the lower end of the plates to the upper portion of the sacrum. In this instance, bone screws are used to connect the brackets to the sacrum.
By providing the plates in a variety of sizes and configurations, the surgeon can quickly and easily immobilize a desired portion of a patient""s spine. Significantly, no drilling is required to install the clamp according to the invention (except for the sacral bracket, if one is used), thereby eliminating the use of pedicle screws, wires, or hooks.